Functional heterogeneity of nociceptin/orphanin FQ receptors revealed by (+)-5a Compound and Ro 64-6198 in rat periaqueductal grey slices

Abstract

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a non-opioid branch of the opioid receptor family implicated in several neurological and psychological disorders, such as pain, anxiety, depression, involuntary movement, addiction, seizure and dementia. Heterogeneity of NOP receptors has been proposed based on the findings of splicing variants and from binding and functional studies. We have previously reported that Ro 64-6198, a NOP receptor agonist, activated a subset, but not all, of N/OFQ-sensitive NOP receptors in midbrain ventrolateral periaqueductal grey (vlPAG). In this study, we found that a new NOP receptor ligand, (+)-5a Compound ((3aS, 6aR)-1-(cis-4-isopropylcyclohexyl)-5'-methyl-2'-phenylhexahydrospiro[piperidine-4,1'-pyrrolo[3, 4-c]pyrrole]), also activated a subset of NOP receptors in vlPAG neurons. (+)-5a Compound (0.1-30 μm) concentration-dependently activated G-protein-coupled inwardly-rectifying potassium (GIRK) channels mediated through the NOP receptors in about 35% of the recorded vlPAG neurons. (+)-5a Compound (EC50: 605 nm) was less potent (1/12) and efficacious (47%) than N/OFQ. In (+)-5a Compound-insensitive neurons, Ro 64-6198 was also ineffective, and vice versa, but N/OFQ activated GIRK channels through NOP receptors. In (+)-5a Compound-sensitive neurons, (+)-5a Compound precluded the effect of Ro 64-6198. Immunofluorecent and morphometric studies showed that most of the (+)-5a Compound-sensitive neurons were multipolar with intensive dendritic arborization and immunoreactive to glutamic acid decarboxylase-67. It is suggested that (+)-5a Compound activates a subset of NOP receptors, similar to the Ro 64-6198-sensitive subset, in the vlPAG neurons which are mostly GABAergic. These results further support the presence of functional heterogeneity of NOP receptors in the midbrain PAG.

Quantitative study of [Tyr10]nociceptin/orphanin FQ (1-11) at NOP receptors in rat periaqueductal gray and expressed NOP receptors in HEK293 cells

“…This phenomenon appears to be similar to the results obtained with (+)-5a Compound (Liao et al, 2011b) and Ro 64-6198 (Chiou et al, 2004), which also affected the NOP receptors in only a portion, but not all, of vlPAG neurons. However, the higher the concentration of [Tyr 10 ]N/OFQ(1-11) tested, the fewer the insensitive neurons (Fig.…”

“…All reasonable efforts were made to minimize the number of animals used. The preparation of midbrain periaqueductal gray slices, electrophysiological recordings and data analysis was similar to our previous study (Liao et al, 2011a(Liao et al, , 2011b.…”

“…The reversal potential of [Tyr 10 ]N/OFQ(1-11)-induced current was −92.4 ± 1.8 mV (n = 40), which corresponds to the equilibrium potential of potassium ions (− 91 mV) according to the Nernst equation. Therefore, in vlPAG neurons, [Tyr 10 ]N/OFQ(1-11), like N/OFQ (Liao et al, 2011a(Liao et al, , 2011b, activated IRK channels which are coupled to Gprotein (Ikeda et al, 1997).…”

“…Previously, we have demonstrated that NOP receptors are functionally heterogeneous in rat ventrolateral periaqueductal gray (vlPAG) neurons using Ro 64-6198 and (+)-5a Compound, two non-peptide NOP agonists (Chiou et al, 2004;Liao et al, 2011b). Both compounds were ineffective in one subset of NOP receptors while N/OFQ affected NOP receptors in almost all of recorded vlPAG neurons (Chiou et al, 2004;Liao et al, 2011b).…”

“…Both compounds were ineffective in one subset of NOP receptors while N/OFQ affected NOP receptors in almost all of recorded vlPAG neurons (Chiou et al, 2004;Liao et al, 2011b).…”